As far as binding crouch/prone to fire, just double bind crouch button onto the left mouse or right mouse.. not hard at all. In GOW3 I double bind spot onto my fire key. The only problem I guess would be then you would have to hold down fire button and every time you press fire you will crouch/uncrouch. TO actually have a seperate key for crouch and prone is impossible because crouch and prone are attached to the same button..
Wouldn't recommend it personally because then you will always be going prone even if you don't want too, just use ctrl lol.